A saponified ethylene-vinyl ester copolymer product (hereinafter sometimes referred to as “EVOH resin”) has a very strong intermolecular force owing to hydrogen bonding of the hydroxyl groups each other present at polymer side chains. Therefore, since the saponified product has high crystallinity and high intermolecular force even in amorphous portions, it is difficult for gas molecules and the like to pass through the EVOH resin and thus the resin exhibits excellent gas barrier properties.
The EVOH resin is produced by saponifying an ethylene-vinyl ester copolymer obtained by copolymerizing ethylene and a vinyl ester monomer.
The vinyl ester monomer to be a raw material of the EVOH resin, for example, vinyl acetate is usually made into a product by reacting ethylene, oxygen, and acetic acid in the presence of a catalyst and purifying the resulting crude product by distillation or the like.
Since vinyl acetate is extremely highly reactive, it is prone to undergoes radical polymerization by heat and further the presence of a metal ion or a peroxide. As a result, heat efficiency is lowered and a transportation pipe is occluded in some cases by attaching and depositing a polymerization product on the walls of a production tank, a storage tank, a distillation column, and a transportation pipe.
Therefore, at the time when vinyl acetate is subjected to polymerization, it is necessary to suppress polymerization during the storage and the like of vinyl acetate and various methods have been proposed.
For example, Patent Document 1 describes that, in order to prevent adverse effects such as polymerization inhibition by oxygen before initiation of the polymerization of a vinyl ester monomer, it is desirable to remove oxygen as far as possible, specifically to control oxygen concentration to 10 ppm or less, preferably 0.1 ppm or less, in a polymerizable mixture (reaction solution etc.) containing the vinyl ester monomer.
That is, since oxygen acts as a polymerization initiator of the vinyl ester monomer, it is a common general technical knowledge that it is preferable to increase the oxygen concentration to such a degree that the vinyl ester monomer is not polymerized, e.g., such a sufficiently high degree as more than 10 ppm at storage or transportation and reduce the oxygen concentration to such a degree that the polymerization of the vinyl ester monomer is not inhibited, e.g., 10 ppm or less at polymerization.